1. Field of the Invention
The present invention relates to a document feeder to be applied to an image processing apparatus such as a copying machine, a facsimile machine or an image reader.
2. Description of Related Art
Automatic document feeders are conventionally known which are applied to copying machines and the like and adapted to separate document originals set on a document placing plate on a one-by-one basis by a separator mechanism and feed the separated document original to a copying position on a platen glass through a document transportation path. The document feeder of this type typically employs a transportation path 320 curved with a predetermined curvature radius as shown in FIG. 6. In this case, a pair of registration rollers 321 responsible for secondary document feeding are provided in an abutting relation at a vertex 322 of the curvature of the transportation path 320. In other words, the pair of registration rollers 321 are provided in such a manner that a plane 323 including axes 321a and 321b of the respective rollers is generally parallel to a surface of a contact glass 324.
Recently, there has been an increasing tendency to reduce the height of an automatic document feeder in order to meet the demand for the size reduction of a copying machines. This involves the reduction of the curvature radius of the document transportation path 320 which, in turn, has a sharper curvature. Therefore, a document original P should be greatly curved as shown by a solid line in FIG. 6 to ensure that the document original can smoothly be guided to the curvature vertex 322 at which the pair of registration rollers 321 abut against each other.
In the prior-art document feeders, however, the document original P approaching the curvature vertex 322 is subject to a transportation force only from a separator mechanism 325. Such a transportation force alone is insufficient for bending the document original P to conform to the sharp curvature of the document transportation path. Hence, when the document original P is fed in a virtually unbent state into the document transportation path 320 as shown by a two-dot-and-dash line in FIG. 6, the document original may bump against an outer interior wall 320a of the document transportation path 320 before reaching the abutment position of the pair of registration rollers 321 located at the curvature vertex 322. As a result, document feeding failures such as a document transportation delay is more likely to occur in primary document feeding.
Since the pair of registration rollers 321 are located at the curvature vertex 322, the document original P is reversed for the secondary feeding thereof during the transportation thereof. At this time, the pair of registration rollers 321 are required to transport the curved document original P in the course of the reversal thereof, so that the document original P is forcibly pulled by the registration rollers 321. Therefore, a relatively large driving force is required for driving the pair of registration rollers 321, resulting in a heavy load on a motor for driving the pair of registration rollers 321.
This phenomenon is particularly noticeable in the case where a thick and stiff document original (such as having a stiffness of 160 g/m.sup.2 or 200 g/m.sup.2) is fed.